The present invention is directed to a hammer drill incorporating an electropneumatic impact mechanism including a guide cylinder or tube for transmitting rotational movement to a tool bit. A rotatable but not axially displaceable gear wheel is provided in permanent meshed engagement with a drive wheel arranged on the guide cylinder. A coupling sleeve is fitted on the guide cylinder so that it rotates with the cylinder, however, it is axially displaceable via a switching device located on the hammer drill housing. The coupling sleeve can be placed in meshed engagement with or disengagement from the gear wheel and the switching device is connected to the coupling sleeve.
In addition to percussion drills, hammer drills are often used for chiseling or cutting operations. In performing such chiseling operations, the rotational movement of the hammer drill is cut off so that the tool bit receives only axially directed blows. To prevent any unintentional rotation of the tool bit in the hammer drill, it has been known to lock the guide cylinder in a specific position. Depending on the type of tool bit holder used, a rotational lock of the guide cylinder is advantageous for use in changing the tool bit.
As a result, in a known hammer drill, an axially displaceable coupling sleeve is used on the guide cylinder or tube so that the sleeve does not rotate relative to the guide tube. The outside surface of the coupling sleeve has a polygonal shape. The switching device in engagement with the coupling sleeve is in the form of an eccentric shaft and serves, on one hand, for the axial displacement of the coupling sleeve, and, on the other hand, for the rotational locking of the coupling sleeve by bearing against one of the flat or planar faces of the polygonally shaped outside surface. Accordingly, the lockable rotational positions of this arrangement are limited by the polygonal shape of the coupling sleeve. If the outside surface of the sleeve forms a guadrilateral, then there are four different possible rotational positions. If the outside surface is a hexagon, then there are six different rotational positions. The number of flat sides of the polygonally shaped coupling sleeve cannot be increased in a random manner, since with an increase the contact surface for the eccentric shaft is shortened and effective rotational locking is not assured. Particularly where the chiseling operation is carried out at an inclined surface, it is advantageous if the attitude of the chisel bit is adapted to the position of the device which is optimal for the operator.